Method for antenna verification

ABSTRACT

A method of enabling a communications receiver to verify an antenna weight previously signalled by the communications receiver to a base station, the method including the steps of; equalizing ( 14 ) a channel estimate of a dedicated pilot channel by the complex conjugate of a channel estimate of a common pilot channel to form an estimate of the transmission weight used by the base station; and for each transmission slot, combining ( 20 ) a component of the transmission weight estimates for current and previous slots to form an optimised transmission weight estimate.

The present invention relates generally to wireless telecommunicationssystems, and in particular to methods of verifying an antenna weightpreviously signalled by a communications receiver to a base stationwithin wireless telecommunications systems.

The present invention has a particular application in systems conformingto the third generation wide band code division/multiple access (W-CDMA)systems, and it will be convenient to describe the invention in relationto that exemplary, but non limiting, application. With this in mind, the3GPP term “User Equipment” will be used to refer to the communicationsreceiver throughout the description of the preferred embodiments.However, the present invention should not be considered to be limited touse with communications receivers operating according to standardsdefined by the 3GPP.

Current W-CDMA standards define the use of closed loop, feed back modetransmitter diversity. FIG. 1 shows a general W-CDMA transmitterstructure to support closed loop mode transmit diversity for dedicatedphysical channel (DPCH) transmission. The DPCH is spread and scrambled.The spread complex valued signals are then fed to two transmitterantenna branches Ant₁ and Ant₂, and weighted with antenna-specificweight factors w₁ and w₂ respectively. The weight factors are complexvalues and correspond to the phase-offset of the two antennas pathsdetermined by user equipment 2 and signalled to base station 1 via anuplink dedicated physical control channel (DPCCH). The uplink DPCCH isused to carry control information consisting of known pilot bits tosupport channel estimation, downlink power-control, feedback information(FBI) and an optional transport format combination indicator. FIG. 2shows the frame structure of an uplink DPCCH. Each radio frame has alength of 10 ms and is split into 15 slots, each of length 2560 chips,corresponding to one power-control period.

The user equipment 2 uses the CPICH to separately estimate channels seenfrom each antenna Ant₁ and Ant₂. Once every slot, the user equipment 2chooses an optimum weight from amongst a mode specific transmit weightset. The optimum weight is selected so that, when applied at the basestation 1, the received power is maximised at the user equipment 2. Theuser equipment 2 then feeds back to the base station 1 FBI bits whichinforms the base station 1 of which power or phase settings should beused.

The user equipment 2 will typically wish to utilise CPICH channels inchannel estimation due to the higher transmission power resulting in amore reliable channel estimation. In order to properly combine thechannel estimates corresponding to CPICH₁ and CPICH₂, the user equipment2 must know the transmission weight that was used by the base station 1.However, the feedback channel that is used to transmit the FBI bits fromthe user equipment 2 to the base station 1, and which are employed bythe base station 1 to steer that phase or amplitude shift of theantennas Ant₁ and Ant₂, are subject to error. Accordingly, the basestation 1 may not always transmit the DPCH using the optimalphase-offset or amplitude-offset determined by the user equipment 2.Without the knowledge of the actual phase-offset or amplitude-offsetused by the base station 1, the user equipment 2 will perform anincorrect channel estimate, resulting in a degradation of the userequipment performance.

Techniques exist to enable the user equipment 2 to estimate the antennaspecific weight factors w₁ and w₂. However, existing techniques areinvariably prone to error and unreliable and/or require expensive andpower consuming computational resources. There currently exists a needto provide a method of enabling user equipment to verify an antennaweight previously signalled by the user equipment to a base station thatis simple, efficient and minimises the resources required to carry outthe method by user equipment.

With this in mind, one aspect of the present invention provides a methodof enabling a communications receiver to verify an antenna weightpreviously signalled by the communications receiver to a base station,the method including the steps of;

equalising a channel estimate of a dedicated pilot channel by thecomplex conjugate of a channel estimate of a common pilot channel toform an estimate of the transmission weight used by the base station;and

for each transmission slot, combining a component of the transmissionweight estimates for current and previous slots to form an optimisedtransmission weight estimate.

The transmission weight may include any one or more of a phase-offset,amplitude-offset or like quantity.

The base station may include two or more antennas for transmitdiversity.

The communications receiver and base station may form part of a W-CDMAor like wireless communication system.

Preferably, the component of the transmission weight estimates that iscombined may depend upon an uplink slot number of the last feedbackinformation to effect the current transmission weight.

The method may further include the step of;

delaying the equalization step; and

forming the optimised transmission weight estimate from a component ofthe transmission weight estimates for current, previous and futureslots.

In embodiments of the invention in which the communications receiver andbase station form part of the W-CDMA system, the in-phase component ofthe transmission weight estimates are combined when the uplink slotnumber of the last feedback information bit is an odd number.

In embodiments of the invention in which the communications receiver andbase station form part of the W-CDMA system, the quadrature phasecomponent of the transmission weight estimates are combined when theuplink slot number of the last feedback information bit is an evennumber or zero.

Another aspect of the invention provides a communications receiveradapted for communication with a base station, including processingmeans for enabling the communications receiver to carry out the abovedescribed method.

The following description refers in more detail to the various featuresof the present invention. To facilitate an understanding of theinvention, reference is made in the description to the accompanyingdrawings where the method of enabling user equipment to verify anantenna weight is illustrated in a preferred embodiment. It is to beunderstood that the invention is not however limited to the preferredembodiment illustrated.

In the drawings:

FIG. 1 is a schematic diagram of an illustrative base station and userequipment forming part of a W-CDMA communication system;

FIG. 2 is a schematic diagram of an uplink dedicated control channel inwhich feedback information is transmitted each slot from the userequipment to the base station of FIG. 1; and

FIG. 3 is a flow chart depicting steps performed by the user equipmentof FIG. 1 in order to verify an antenna weight previously signalled bythe user equipment to the base station of FIG. 1.

In order to assist in the clarity of the following description, thephase-offset by the base station 1 shall be referred to as the “truephase-offset”, while the phase-offset that the user equipment 2 signalsto the base station 1 shall be referred to as the “intendedphase-offset”. It will be appreciated that whilst phase-offset is usedin this exemplary embodiment as an example of an antenna weight set bythe base station 1, in other embodiments of the invention the antennaweight may be an amplitude-offset or other quantity.

The true phase-offset is the same as the intended phase offset when thebase station 1 receives both feed back information (FBI) bits withouterror. However, when one or more of the two FBI bits are in error thetrue phase-offset will be different from the intended phase-offset.During soft handover, base stations that receive FBI bits in error willset a different phase-offset to other base stations, so that antennaverification procedure should be applied to each radio link separately.

The user equipment 2 estimates which possible phase-offset value has thehighest probability of being the true phase-offset. To assist in theclarity of the following explanation, the phase-offset selected by theuser equipment 2 shall be referred to as the “hard phase-offsetestimate”. The estimates of the channels seen from antenna Ant₂ arerotated by the hard phase-offset estimate before the DPCH symbols areequalized.

As can be seen in FIG. 3, the user equipment 2, at step 10, demodulatesthe DPCH pilot symbols with the pilot symbol pattern for antenna Ant₂.At step 12, and the sum of the demodulated pilot signals is determined.This sum is a DPCH based channel estimate for the antenna Ant₂ channelthat includes the true phase-offset. The antenna Anti component of theDPCH is not present in this channel estimate because the pilot symbolpatterns for antenna Ant₁ and antenna Ant₂ are orthogonal.

At step 14, the user equipment 2 acts to equalise the DPCH based channelestimate for the antenna Ant₂ channel with the complex conjugate of theCPICH channel estimate for antenna Ant₂. This acts to remove the phaserotation of that channel.

At step 16, the result is summed across all fingers allocated to thesame radio link to form an estimate of the true phase offset. In thefollowing description, this estimate shall be referred to as the “softphase-offset estimate”.

Alternatively, the DPCH pilot symbols and the CPICH channel estimatescan be combined (see step 18) across all fingers allocated to the sameradio link, and these combined symbols used in the demodulation andequalisation steps 10-16 to form the soft phase estimate for the radiolink. Which method is used will depend upon the implementation of therake receiver (not shown) used in the user equipment 2.

The soft phase estimates for each finger “f” of a same radio link, ascalculated by the user equipment 2 in steps 10-14 is given by thefollowing equation:

$W_{f} = {\left\lbrack {\sum\limits_{i = 0}^{N_{pilot} - 1}\left( {{R_{i,f} \cdot X}\; 2_{i}^{*}} \right)} \right\rbrack \cdot C_{2,f}^{*}}$

Where:

-   -   R_(i,f) represents the received DPCH pilot symbols on finger ‘f’    -   X2 _(i)* represents the complex conjugate of the antenna 2 pilot        symbol pattern    -   N_(pilot) is the number of pilot symbols in the slot C_(2,f) is        the complex conjugate of the antenna 2 channel estimate for        finger ‘f’    -   W_(f) is the soft phase-offset estimate for finger ‘f’

The soft phase-offset estimates summed in step 16 across all validfingers allocated to the same radio link as given by the equation:

$W = {\sum\limits_{f}W_{f}}$

The user equipment 2 uses the soft phase estimates calculated in steps10-16 in order to derive the hard phase-offset estimate. Whilst otheralgorithms have been proposed in this area, none have been found to usethe soft phase-offset estimates as efficiently or produce hardphase-offset estimates with as low an error rate.

At each slot, either the in-phase or the quadrature component of thetrue phase-offset has the same value for the current slot and a previousslot. At step 20, the user equipment 2 acts to combine this component ofthe current and previous soft phase-offset estimates in order todecrease the error rate of the hard phase-offset estimate.

The component of the soft phase-offset estimates that is combineddepends upon the uplink slot number of the last FBI bit to effect thecurrent true phase-offset. The 3 different possible cases in W-CDMAsystems are: even uplink slot numbers except for slot numbers 0, odduplink slot numbers and uplink slot number 0. The user equipment 2 isadapted to determine the uplink slot number of the last FBI bit toeffect the current slots true phase-offset using the current slotdownlink number and the closed loop timing mode (J+1 or J+2).

Table 1 shows which component of the true phase offset is combinedaccording to the uplink slot number of the last FBI bit to effect thecurrent transmission weight in a W-CDMA system.

Hard Phase-Offset Estimate Based on Soft Phase-Offset Estimates NotUsing the Next Slot

TABLE 1 Uplink Slot Number Slot 2, 4, 6, 8, 10, 12 W_(I) = W_(I) ^(i)W_(Q) = W_(Q) ^(i) + W_(Q) ^(i−1) or 14 Slot 1, 3, 5, 7, 9, 11 W_(I) =W_(I) ^(i) + W_(I) ^(i−1) W_(Q) = W_(Q) ^(i) or 13 Slot 0 W_(I) = W_(I)^(i) W_(Q) = W_(Q) ^(i) + W_(Q) ^(i−1) + W_(Q) ^(i−2)

In table 1, W_(I) ^(i) and W_(Q) ^(i) are the in-phase and quadraturecomponent of W for the current slot,

W_(I) ^(i−1) and W_(Q) ^(i−2) are the in-phase and quadrature componentof W for the previous slot, and

W_(I) ^(i−2) and W_(Q) ^(i−2) are the in-phase and quadrature componentof W for the slot before the previous slot.

For odd slot numbers, the in-phase component of the soft phase-offsetestimate for the current slot and the previous slot are combined sinceit is known that the in-phase component of the true phase-offset must bethe same for these slots.

Once per frame there are three slots in a row with the same quadraturecomponent of the true phase-offset. This occurs because of the two evennumbered slots in a row, i.e. slot 14 followed by slot 0 of the nextframe. Extra benefit is gained from combining the quadrature componentof all three soft phase-offset estimates when the current slot number is0.

If the equalisation of the data symbol with the channel estimate can bedelayed in the base station 1, the soft phase-offset estimate of afuture slot can also be used. This will further decrease the error rateof the hard phase-offset estimate.

Table 2 shows those components of the current, previous and future slotsthat are combined in such a case.

Hard Phase-Offset Estimate Based on Soft Phase-Offset Estimates Usingthe Next Slot

TABLE 2 Uplink Slot Number Slot 2, 4, 6, 8, 10, 12 W_(I) = W_(I)^(i+1) + W_(I) ^(i) W_(Q) = W_(Q) ^(i) + W_(Q) ^(i−1) or 14 Slot 1, 3,5, 7, 9, 11 W_(I) = W_(I) ^(i) + W_(I) ^(i−1) W_(Q) = W_(Q) ^(i+1) +W_(Q) ^(i) or 13 Slot 0 W_(I) = W_(I) ^(i+1) + W_(I) ^(i) W_(Q) = W_(Q)^(i) + W_(Q) ^(i−1) + W_(Q) ^(i−2)

In table 2, W_(I) ^(i+1) and W_(Q) ^(i+1) are the in-phase andquadrature component of W for the next slot.

From the combined soft phase-offset estimate thus calculated, acorresponding hard phase-offset estimate, as shown in table 3, isdetermined by the user equipment 2.

TABLE 3 Combined soft phase-offset estimate Hard phase-offset estimateW_(I) ≧ 0 and W_(Q) ≧ 0  π/4 W_(I) < 0 and W_(Q) ≧ 0 3π/4 W_(I) < 0 andW_(Q) < 0 5π/4 W_(I) ≧ 0 and W_(Q) < 0 7π/4

It will be appreciated from the foregoing that the above describedmethod can be extended to take into account the downlink signal quality,the assume FBI error rate on uplink, or any other relevant information.For example, the probability of the intended phase-offset is higher thanother phase-offsets. Moreover, the variance of the soft phase-offsetestimate is related to the downlink signal quality, so less weight maybe given to the soft phase estimates when the downlink signal quality ispoor, and more weight given when the downlink signal quality is high.Accordingly, the regions may be changed to reflect the a prioriprobabilities of each of the phase-offsets and the estimated variance ofthe soft phase-offset estimates.

Antenna verification cannot be used for closed loop mode 2 (as definedby current 3GPP standards), because the DPCH pilot patterns on the twoantennas Ant₁ and Ant₂ are not orthogonal. However, the above describedmethod could well be extended to closed loop mode 2 if the DPCH pilotpatterns were changed to make them orthogonal. Current 3GPPspecifications described two approaches for deriving a hard phase-offsetestimate from soft phase-offset estimates. The first 3GPP methoddescribes using a four-hypothesis test per slot, with each hypothesisbased on one of the four possible phase-offset values (the four valuesindicated in table 3). This method does not however use the previousslot's soft phase-offset estimate even though the previous and thecurrent slot's true phase-offset must be in the same half-plane. Themethod described in the present specification, in the context of theexemplary embodiment shown in FIGS. 1-3, also uses a four-hypothesistest per slot, but provides a significant improvement due to the softphase estimates of previous slots being used to improve the reliabilityof the hard phase-offset estimate.

The second 3GPP method described in current 3GPP standard uses atwo-hypothesis test per slot, based on two possible received FBI valuesfor the last FBI bit that effects the current slots true phase-offset.In this context, the “received FBI value” is the FBI value detected atthe base station 1 and used to determine the true phase-offset. The bestestimate by the user equipment 2 of the FBI value received by basestation 1 shall be referred to as the “FBI value estimate”. The FBIvalue of a particular downlink slot shall correspond to the lastreceived FBI value that effects the slots true phase-offset, even thoughthe FBI value was sent on an uplink 1 or 2 slots beforehand. Accordingto current 3GPP standards in relation to the second 3GPP method, the FBIvalue estimates from the last even and last odd numbered uplink slotdetermine the hard phase-offset estimate. It will be noted that if theestimated FBI value in this method is incorrect, and the estimatedphase-offset will be incorrect for at least two slots in a row. Thecurrent 3GPP method does not use both the current and previous slotssoft phase-offset estimate for the previous slots FBI value estimate,even though the true phase-offset of both slots depends on the previousslots received FBI value.

The method described in the present specification is also applicablewithin the context of a two-hypothesis test per slot method. The currentslots soft phase estimate can accordingly be used to revise and improvethe reliability of the previous slots FBI value estimate, thus providinga significant improvement over the existing second 3GPP method.

The antenna weight verification method described in the presentspecification is applicable to closed loop transmit diversity systems,or other communications systems, where a component of the phase-offsetor amplitude-offset between antennas, or other quantity, is maintainedover multiple periods. Components of the soft estimates that aremaintained over multiple periods are soft combined to obtain a morereliable estimate of the phase-offset, amplitude-offset or otherquantity. The invention has been described in the present specificationin the context of antenna verification for closed loop mode 1 transmitdiversity in a 3GPP W-CDMA system including base station and userequipment, however it will be appreciated that this is but one exemplaryapplication of the invention and that the invention is also applicableto other technologies and systems including different types ofcommunication receivers and network apparatus.

1. A method of enabling a communications receiver to verify an antennaweight previously signalled by the communications receiver to a basestation, the method including the steps of: equalising a channelestimate of a dedicated pilot channel by the complex conjugate of achannel estimate of a common pilot channel to form an estimate of thetransmission weight used by the base station; for each transmissionslot, combining a component of the transmission weight estimates forcurrent and previous slots to form an optimised transmission weightestimate; delaying an equalization of data symbol with the channelestimate in the base station; and forming the optimised transmissionweight estimate from a component of the transmission weight estimatesfor current, previous and future slots, wherein the communicationsreceiver and base station form part of a W-CDMA wireless communicationsystem, and the component of the transmission weight estimates that iscombined depends upon an uplink slot number of the last feedbackinformation bit to effect the current transmission weight.
 2. A methodaccording to claim 1, wherein the transmission weight includes any oneor more of a phase-offset or amplitude-offset.
 3. A method according toclaim 1, wherein the base station includes two or more antennas fortransmit diversity.
 4. A method according to claim 1, wherein thein-phase component of the transmission weight estimates are combinedwhen the uplink slot number of the last feedback information bit is anodd number.
 5. A method according to claim 1, wherein the quadraturephase component of the transmission weight estimates are combined whenthe uplink slot number of the last feedback information bit is an evennumber or zero.